Link adaptation usually refers to the ability to adapt a modulation and coding scheme, MCS, used for transmission of data to match channel conditions of a link used for the transmission. If a too high MCS is used (i.e. a too high modulation and/or a too high code rate), the error rate is likely to be too high, whereas if a too low MCS is used (i.e. a too low modulation and/or a too low code rate), the spectrum usage is likely to be unnecessarily low. Thus, using the most suitable MCS is highly desirable to make efficient use of the spectrum resources with a low error rate in most wireless communication systems.
In order for a transmitter to select a suitable MCS, the transmitter needs some kind of feedback from the receiver regarding the channel conditions. This feedback may be explicit, i.e., the receiver side may send back channel quality information, CQI, which can be used to determine the most suitable MCS on the current channel used, or the receiver may even send back information regarding which MCS estimate is the most suitable one for the transmitter to use on the current channel used. The feedback may also be implicit, e.g. the feedback may only be that correctly received packets are acknowledged whereas nothing is sent back in case the packet is not correctly received. In this latter case, the transmitter has to determine what a suitable MCS is based on the statistics of the correctly received packets, usually the packet error rate, PER. For example, if the PER is close to 0%, the transmitter may choose to increase the MCS, whereas if the percentage of correctly received packets is too small, the transmitter may choose to decrease the MCS.
In 802.11 standard, e.g. 802.11n and latest 802.11ac, the MCS and multiple input multiple output, MIMO, mode (either space time block code or spatially domain multiplexing) are calculated and recommended at terminals, herein referred to as stations, STA, side. The standard does not specify the technique by which the STA derives an MCS recommendation. Usually the recommendation criterion is to optimize the throughput or to guarantee sufficiently low PER to avoid unnecessary retransmission based on link quality. The MCS feedback field in the High Throughput, HT, Control field is used as a means for providing this feedback. The information of recommended MCS and MIMO mode is signaled to the network node, herein referred to as the access point, AP, through the MCS feedback field in the HT Control field. At the AP side, the AP combines the information about recommended MCS and other information it already has (e.g. transmit power amplifier backoff) to derive the final MCS and then the packet size for a down-link, DL, transmission.
One issue with link adaptation is that it requires the channel to be sufficiently slow varying. Specifically, the channel conditions for the next packet to be transmitted must be possible to determine so that a suitable MCS can be selected based on the earlier estimated channel conditions. In case of explicit feedback, the CQI provides, as such, limited value unless the transmitter uses the same settings as was used when the receiver generated the explicit feedback. In particular, if the transmission is done at another frequency channel, with potentially very different channel conditions, an MCS suggested by the receiver may be completely inadequate for the transmitter. Alternatively, if the CQI is varying very quickly due to e.g. intermittent interference, any link adaptation algorithm may have difficulty finding the optimum MCS.
In case of implicit feedback using packet error rate, PER, used in Wi-Fi, e.g. 802.11g, 802.11n, and 802.11ac, the issues may be even worse. Since PER is measured over a long period compared with the duration of one packet, link adaptation based on PER adapts slowly to time-varying channel conditions.
Orthogonal frequency division multiple access, OFDMA, has been adopted in LTE systems, where link adaptation including MCS selection and multiple user, MU, scheduling is performed simultaneously based on the CQI feedback from terminals. The goal is to maximize the overall throughput in the DL. Hence, the procedures of MCS selection and MU scheduling are coupled to optimize the overall performance. In the DL signaling, the information of the frequency resource allocation and MCS are carried in respective fields contained in DL. However, the link adaptation method used in LTE systems and existing optimization algorithms used for joint optimization may not be applicable to systems operating in an unlicensed band such as wireless local area network, WLAN, systems. In unlicensed band systems, MCS selection is mainly based on acknowledgement, ACK, and non-acknowledgment, NACK, and hence, it is not feasible to perform joint MU scheduling and link adaptation at AP. MU scheduling and link adaptation, to some extent, are decoupled.
In both uplink and downlink transmissions, the network node schedules different nodes, herein referred to as stations, STAs, in a part of the channel that has relatively good channel condition. However, as the channel changes with time, and the channel allocation to the different users typically varies with time, a link adaptation algorithm that is based on feedback from STA would typically have difficulties to perform timely adaptations. In such an OFDMA-based WLAN system, the AP is responsible for user allocation in the DL. Link adaption may still be based on the STA recommended MCS. In this procedure, when quality of the sub-carriers/sub-channel allocated to a certain STA degrades, the AP allocates a new link/sub-channel with possibly better quality. However, there is a considerable delay before the appropriate MCS for the new link/sub-channel is in place as the STA cannot know in advance what MCS is to be selected.
There is therefore a need for a mechanism that provides an efficient MCS selection that prepares for a sudden change in channel condition such as the sudden changes characterizing unlicensed bands.